Electron multiplier and method of making same

ABSTRACT

Dynode plates useful in electron multiplying devices and method of forming same, which comprises providing a plate of insulating material having a plurality of holes therethrough, glazing a layer of a different material such as lead glass on the inside of the holes, and subjecting the glazed material to conditions whereby at least the surface of the glazed material becomes semiconductive.

Unlted States Patent 1 [in 3,911,167

Linder Oct. 7, 1975 l l ELECTRON MULTIPLIER AND METHOD OTHER PUBLICATIONS OF MAKING SAME [75] Inventor: Jacques Under Palos Verdes Blodgett, R. B. Electrically Conducting Glasses,

peninsula Call-f Chemical Abstracts, Vol. 42. p. 3921(a) l948.

[73] Assignee: Texas Instruments Incorporated,

Dallas Tex Primary Examiner-Mayer Weinblatt Attorney, Agent, or FirmHarold Levine; James T. [22] led: 1974 Comfort; Richard L. Donaldson Appl. No: 462,466

Related US. Application Data II I? [57] ABSTRACT Dynode plates useful in electron multiplying devices and method of forming same, which comprises providing a plate of insulating material having a plurality of holes therethrough, glazing a layer of a different material such as lead glass on the inside of the holes, and subjecting the glazed material to conditions whereby at least the surface of the glazed material becomes semiconductive.

4 Claims, 3 Drawing Figures U.S. Patient Oct. 7,1975 3,911,167

FIG.2

I N FIG.3

INVENTOR JACQUES F LINDER SOKOLSKI & WOHLGEMUTH ELECTRON MULTIPLIER AND NIE'I'HOD OF MAKING SAIVIE This is a continuation of application Ser. No. 33,830, filed May 1, 1970.

In US. Pat. No. 3,408,532 of Oct. 29, 1968, there is disclosed an electron beam scanning device which is comprised of a plurality of dynode plates of an insulative type material. The dynode plates have a plurality of holes formed therein which holes are coated with a material that has secondary emissive characteristics. The herein invention particularly relates to an improved method for forming the secondary emissive coating in the holes of the dynode plates. There have additionally existed other forms of electron multiplier devices which utilize insulative plates having apertures coated with secondary emissive material. Thus, the herein invention also pertains to a method of forming such plates regardless of the type of device in which they are to be utilized.

Prior to the present invention various attempts have been made to successfully coat holes in an insulator plate with secondary emissive material. One of the most conventional approaches involves sputtering a material, such as tin oxide. The sputtering is done from each side of the plate toward the center of the hole. If the hole is quite deep, the coating might not reach the center of the hole. Further, sputtering does not provide as even a coating as is desired. Since the resistance of the coating varies as to its thickness an uneven coating provides uneven results within the hole.

A different approach, which is more closely related to the present invention, involves the utilization of a plate of a lead glass material which had holes etched therethrough. A plurality of the plates are then stacked and bonded together with the holes co-aligned. Next the plates are subjected to a reducing procedure, which comprises heating the array and flowing hydrogen through the channels. This converts the surface of the holes to a semiconductive material. In other words, a surface layer of the lead glass in the hole becomes resistive after the reducing treatment. There are several disadvantages to this prior technique. Firstly, the technique is limited to the utilization of the lead glass for the basic dynode insulative sheet. It is undesirable for the insulative sheet to be made of the lead glass for various reasons. The lead glass is very unstable against chemical attack and thus the utilization of the sheet in certain atmospheres could result in deterioration thereof. Additionally, lead glass is mechanically fragile and thus cannot be subjected to environments where it might be readily jarred. Not only is the lead glass mechanically fragile, but it also does not resist thermal shock and can crack when subjected to extremes of temperature.

In view of the foregoing, devices where the entire dynode sheet or plate is comprised of lead glass cannot be used in a variety of environments. The sheets cannot be readily handled or transported without taking great pains to prevent their breaking. A still further disadvantage of the lead glass plates is that to date there has not been a successful method for providing accurate holes through thick plates. Typically in the past the maximum ratio of the length of the hole to its diameter in such lead glass plates has been about 5 to I. This requires a greater number of plates in a given array, and furtherlimits the design of a device to plates having a short hole therethrough. For example, it is particularly desirable to have a ratio of hole length to diameter of at least 10 to l for various applications.

Thus it is an object of this invention to provide a dynode plate and its method of manufacture which can utilize any desired insulative material through which holes can be readily and accurately formed.

Still another object of this invention is to provide a high-strength dynode plate and method of making same.

A further object of this invention is to provide a dynode plate and method of making same which can resist thermal shock.

One further object of this invention is to provide an aperture dynode plate where an even layer of resistive secondary emissive material can be provided in the apenures.

The above and other objects of this invention are accomplished by dynode plates and a method of making same which involves the utilization of any suitable insulative material as the basic plate. The insulative material used as the plate should preferably be one in which accurate holes can be formed therein. Particularly preferred are ceramic materials which have photosensitive characteristics which allow the holes to be so accurately formed. The holes or apertures in the plate are then subsequently coated with a glaze of a composition which can be subsequently treated so as to form a semiconductive secondary emissive layer in the holes. For example, the holes can be glazed with a particular lead glass composition which can be subjected to a hydrogen reducing atmosphere to convert an exposed layer of the glass in the holes to a semiconductive and secondary emissive surface.

In one of the novel aspects of the invention, the composition of the glaze is adjusted so that it will match the thermal expansion of the substrate material. To achieve the glaze coating in the holes the insulative substrates are dipped into a liquid dispersion containing fine particles of the material to be glazed, thus coating the substrates. The liquid is then evaporated from the coated substrates, leaving the fine particles of the material to be glazed thereon. This is followed by removing the particles by brushing or the like from the flat sides of the substrate such that particles remain only within the holes. Next, the substrate is subjected to a temperature sufficient to melt and glaze the particles in the holes.

It is believed that the invention will be better understood from the following detailed description and drawings, in which:

FIG. 1 is a partial cross-sectional view of an insulated substrate prior to coating with the material of this invention.

FIG. 2 is a cross-sectional view of the substrate of FIG. 1 entirely coated with a layer of fine particles of the material to be glazed.

FIG. 3 is a cross-sectional view of the substrate of FIG. 2 after the material has been glazed only in the holes.

Turning now to FIG. 1 there is seen a substrate 1 l of an insulative material which has a plurality of holes 13 formed therein. Sometimes the holes 13 are angularly disposed at an angle of up to 20. This corresponds with the type of dynode plates shown in the aforementioned US. Pat. No. 3,408,532. It should be pointed out, however, that the invention is suitably applied to plates with the holes being perpendicular to the surface thereof. The substrate material chosen should preferably have good thermal and strength characteristics. Particularly it should be able to sustain the temperature of glaze re quired to coat the holes 13. Additionally, the substrate should be susceptible to forming the small holes 13 therein which can vary from 50 to 500 holes per inch. One of the main problems in forming the dynodes is in obtaining definition in the small holes utilized. Thus the substrate should be of a material that allows one to form such holes with excellent definition. A major advantage of the present invention is that one can choose the substrate that has the best definition in its apertures provided it meets the other characteristics.

Another point to be kept in mind in choosing the substrate for the invention is that it should not modify the composition of the coating to be used in the holes. For example, some substrates would tend to react with the material to be coated in the holes and prevent it from being converted to a semiconductive or secondary emissive material. One of the most preferred substrates to be utilized is known as Fotoceram, which is a photosensitive crystalline ceramic that has been previously utilized in electronic art. This material is made by the Coming Glass Works Company and meets the requirements set forth above for a good substrate material. However, other various ceramics can be utilized as well as various high-strength glasses, in which well defined holes can be formed.

Once the substrate has been provided with the aper tures 13, it is ready to have these apertures coated with a semiconductive material. As indicated, the process of the herein invention involves coating the apertures 13 with a glaze of a material that can be subsequently treated so that it will become semiconductive and have secondary emission. The description of the process will be with regard to a lead glass composition. However, other similar compositions, such as bismuth glass, or semiconductive glasses like certain iron oxide glasses, vanadium glasses, uranium glasses, strontium and titanium glasses, or chalcogenide are contemplated and can be used. As pointed out, it is particularly desirable that the composition of the material to be glazed in the holes has a thermal expansion which matches that of the substrate. Further, the material used in the glaze should not be reactive with the substrate. Thus, it has been found that the lead glass composition is most suitable for this application.

The main components of the lead glass composition are lead oxide, PbO, silicon dioxide, SiO and boron trioxide, B To these components, bismuth trioxide, Bi O is added in varying amounts to effect the surface resistance of the resulting emissive material. Various other oxides are normally also present in lesser amounts in this particular type of glass composition. in forming the lead glass composition, there is normally a balance between the amount of SiO and B 0 present. The more SiO the higher the melting point of the glass, while alternatively, the more B 0 the lower the melting point. The melting point of the glass composition should be adjusted to a temperature below the deformation point of the substrate on which it is going to be placed. A typical melting point for a desirable lead glass composition can range from 500 to 550C. The boron oxide, B 0 additionally affects the wetability of the glass upon the substrate material. Thus, the amount of this material is once again adjusted to achieve a desired wetting.

The lead glasses are well known and the compositions can vary widely. For example, the amount of lead oxide can vary from 30 to mole percent of the composition. The remaining oxides present, including the boron trioxide, silicon dioxide and the bismuth trioxide among others, will affect various physical properties of the lead glass and bear directly upon its conversion to a semiconductive material when exposed to reducing atmosphere. The lead glasses are generally commercially available. However, these glasses do not normally contain a sufiicient percentage of bismuth trioxide to achieve the desired surface resistance in the final product that is desired. A typical composition of a lead glass suitable in the present invention would comprise the following mole percents: P O 45; B 0 30; SiO 9; CuO 3; ZnO 6.30; Bi O 6; A1 0 0.30; ZR O 0.20; TiO 0. 10, with traces of iron, manganese, calcium, silver and the like amounting to less than 0.10 percent.

Obviously, the glass can be formulated by mixing the desired oxides and heating them to the melting point. However, from a practical standpoint, it has been found desirable to purchase previously made lead glass, melt this glass and add any further materials thereto in the desired amounts. Particularly, it is found that it is preferable to add the bismuth trioxide in the desired amount to the melted commercially available lead glass. The resulting melt is then divided into fine solid particles of the glass. This can be most preferably accomplished by pouring the hot melt into water which is initially at ambient temperature to provide what is known as water frit. The melted glass upon striking the water breaks up into small particles or frit which is the first step in forming the preferred small size particles. The frit is then removed from the water, dried, and placed in a ball mill to which is added a small amount of a suitable liquid that will not attack the glass or the substrate material upon which the glass will be eventually placed. lt is found it is particularly desirable to utilize an organic liquid such as amyl acetate. However, water or other similar types of liquid can be utilized in the ball mill.

The frit is ball milled to a size of particles of less than 5 microns. The ball milling can continue until as low a particle size is obtained as possible. The 5 micron level is found particularly preferable as a maximum size, since it is undesirable for any of the particles to settle within the eventually formed suspension. In other words, with the particle sizes being no greater than 5 microns, a stable suspension is readily obtained. After the ball milling, further liquid is added to obtain a specific gravity of the suspension between 1.0 and 1.30. The reason for obtaining this specific gravity is that if there is not enough liquid present in the suspension, one would plug the holes of the wafer element when it is dipped in the solution. If there is too much liquid, however, there would be too thin a coating. This would result in a discontinuous glaze in the holes of the substrate.

The suspension of the small particles of the glass composition in the liquid is then utilized for coating the substrate. Prior, however, to coating the substrate it must be thoroughly cleaned. Any suitable cleaning technique can be utilized. For example, with a ceramic material, the substrate can be alternately dipped in an acid solution and then in a caustic soda one. For example, the substrate can be dipped repeatedly in hydrochloric or nitric acid solutions of -30% concentration, followed by dipping it in water and then a caustic soda solution of the same concentration. This is followed by several consecutive rinses in water, and drying.

The clean substrate is then dipped into the suspension of the small glass particles. The grit will stick to the substrate by a wetting of the liquid in the suspension and form an entire coating of the grid 15, as seen in FIG. 2. The cleaning of the substrate enables one to achieve the desired wetting action and allow the grit to adhere to the substrate 11. The surface tension of the suspension on the substrate will overcome the gravity effect in the small holes 13 and allow the material to remain therein after the substrate is removed from the suspension. It is preferable that the suspension be well agitated prior to dipping the substrate therein. Thus, an agitator such as an ultrasonic agitator can be utilized to adequately disperse the particles in the liquid. Shortly after the agitation of the liquid, for a period of time to allow it to calm, the substrate is dipped into the beaker to achieve the aforementioned coating.

The coated substrate, as seen in FlG. 2, after being removed from the suspension is then subjected to a drying operation to remove the liquid, allowing only the small particles of glass to remain on the surface. The drying is an important operation since this provides a homogenous coating in the holes 13 of the substrate. The coated substrate is preferably mounted on a fixture which will rotate it slowly in a drying environment. As pointed out, the substrate will often have the apertures or holes 13 at an angular disposition as shown in the Figures. The rotation of the substrate prevents the small particles from settling to the bottom of the holes and allows them to be evenly distributed therein.

Generally the drying will take place where there is a flow of air such as a hood or the like. Successful drying is obtainable at ambient temperatures for most preferred liquids utilized in the suspension. The drying is continued until all the solvent is removed. One can determine this point by the absence of an odor when the solvent utilized has an odor. Alternatively, for many of the glass compositions, there will be a change in color. For example, with a lead glass composition as mentioned above, the coating changes from a green color to an almost white one upon removal of the liquid.

The dried substrate, having the particles of glass thereon, is removed from the drying environment. For most applications, particularly for the type of devices disclosed in the aforementioned US. Pat. No. 3,408,532, the glass particles are removed from the flat surfaces of the substrate material, leaving the glass only in the holes, as seen in FIG. 3. This is accomplished by rubbing the surfaces of the substrate with a foam rubber pad to which can be connected a suction line to slightly draw the particles away from the surface. Any suitable means can be utilized, however, to brush off or remove the particles from the flat surface.

As pointed out in the aforementioned patent, the wafer elements or substrate elements therein have fingers of conductive material selectively applied to the flat surface of the plates, so that particular holes can be selectively energized. If it was desirable to utilize or activate the entire substrate at one time, then there will be no requirement to rub the glass particles ofi' the substrate. Rather, the entire article would be coated with the material. Even in such an embodiment, the present invention is important since it still provides a basic structure having greater strength due to the substrate material than those previously available where the entire article was for example comprised of a lead glass.

The substrate containing the fine glass particles is then placed in an oven and heated to a temperature sufficient to fuse the particles together into a glaze of the glass on the substrate. For a lead glass composition, it has been found that a temperature in the range of 500 to 550C is suitable. Particularly for a lead glass composition as described above, a range of 515 to 520C has been utilized. The point is that the temperature required can vary widely and is dependent of course upon the point where fusion will occur. The time of heating, once again, is determined by that required to achieve the fusion or glaze. The most suitable times and temperatures can be readily determined for a given glass composition by simple trial and error. For the glass composition mentioned above, and at a temperature range of 515 to 520C, 20 minutes of heating has been found sufficient to achieve the glaze.

Where the holes are slanted as shown in the Figures, a two-step process is required to achieve an even glaze coating. The coated substrates are held vertically in the oven during heating. As can be appreciated during the melting and fusing of the particles they tend to accumulate at the lower end of the holes providing a thicker coating at that portion. The substrate is then removed from the furnace, cooled, and again dipped into the suspension of frit, however suspending it vertically from the opposite side to that which was originally utilized for the first dip. The same second side is then used for placing the substrate in the furnace to fuse a second layer of particles.

By so rotating the substrate in the two-step process an even coating in the holes is obtained, even though the holes are slanted. After the wafer is finally removed from the oven it is cooled to room temperature and inspected with a microscope to be sure that none of the holes are blocked. lf a hole is blocked it can be sometimes cleaned out by mechanical punching prior to subjecting it to further processing or the glaze will be stripped away from the wafer by the same acid/caustic cleaning treatment as mentioned above and the wafer entirely re-glazed. The inspected substrate is then placed in an oven where it is heated in a reducing atmosphere to obtain a semiconductive layer on the lead glass. This is a well known approach which has been previously described. The time and temperature of heating as well as the type of reducing atmosphere can vary widely from material to material.

It should be pointed out that a maximum depth of the semiconductive layer is achieved and thus a continued heating in the reducing atmosphere above a certain point is of no value. For example, it has been found with the aforegoing described lead glass composition, a substrate could be heated from between 325 and 350C for about 16 hours in a hydrogen atmosphere. Simple trial and error experiments can be conducted to determine the optimum relationship between temperature and time of heating in a given atmosphere to obtai the desired semiconductive layer in the holes.

Instead of hydrogen any suitable reducing atmosphere can be used. For example, carbon monoxide gas can also reduce the lead glass. The resulting product, as seen in FIG. 2, after being removed from the reducing oven has a glazed coating 17 of the lead glass in each of the holes or apertures 13 of the substrate 11.

A thin semiconductive layer I9 is provided on the surface of each of the holes through the action of the reducing atmosphere. One of the advantages of course in this approach is that even if there are surface irregularities of the glass 17 in the holes, the depth of the semiconductive layer will be constant and thus there will be no variation in the properties of the semiconductive layers through the holes. It is believed that the invention will be further understood from the following detailed example.

EXAMPLE A wafer or substrate of Fotoceram was coated in accord with this invention. The substrate was a 3% inch square having a thickness of H16 inch. There were l28 x 128 holes in the substrate which were to be coated with a semiconductive layer. A commercial lead glass composition was obtained and melted. To this was added bismuth trioxide such that the final composition comprised in mole percent P,,O 45, B 30, SiO 9, CuO 3, ZnO 6.30, Bi- O 6.0, M 0 0.30, Zr O 2.0, and TiO. 0.10, other oxides no more than 0. 10. The melted glass composition was poured into water to form a water frit. One hundred grams of the frit was then added to 250 cc of amyl acetate and placed in a ball mill which contained 7 balls having a total weight of about one pound in a ball mill jar. The frit was then ball milled for [30 hours. At the end of this period the suspension of frit in the amyl acetate was then diluted to a specific gravity of l.02 and poured into a beaker having a diameter of 3% inches by inches deep. An ultrasonic agitator vibrated the suspension in the beaker for one minute prior to dipping the substrate therein.

The substrate prior to dipping into the suspension was carefully cleaned by first dipping it into a 7r solution of nitric acid. The wafer was allowed to remain in the solution which was ultrasonically agitated for five minutes. lt was then removed and washed with water, and then placed in a caustic soda bath for an additional five minutes under ultrasonic agitation. After the caustic bath the substrate was then washed with flowing water and then washed three times in deionized water under ultrasonic vibration. This was followed by drying the substrate by blowing nitrogen thereon. After exposure to the nitrogen gas. it was then fired at 550C for 20 minutes to assure complete removal of any acid de posit and any organic. Ten seconds after the agitator was stopped, the wafer which was held on one edge was dipped vertically into the suspension, completely covering the wafer. Following this the wafer was then pulled vertically out of the beaker and mounted on a fixture which rotated it slowly at the rate of 2 rpm in a laminar flow hood at room temperature. The rotation of the substrate continued for fifteen minutes at which point it was dry due to the evaporation of the amyl acetate from the suspension.

After the liquid in the suspension had been evaporated, the two flat surfaces of the substrate were wiped with a silicon sponge rubber surface to which was connected a vacuum line drawing a slight suction thereon. This served to wipe the frit off of both faces of the substrate leaving it only in the holes.

The wafer was then put in a furnace which was maintained at 550C and left there for 20 minutes. While in the oven, the wafer was held in a vertical position. As indicated above, the frit tended to coat one end of the slanted holes more than the other. At the end of 20 minutes, the wafer was removed from the oven and cooled in air to ambient temperature. It was then rotated such that the opposite side of the wafer was then held so that it could be dipped again into the suspension and the process was repeated exactly, so as to obtain an even thickness on each end of the holes. The cooled wafer after the second treatment was inspected with a microscope to be sure that there was no blockage of holes. It was then placed in a reducing oven. A typical such oven is made of a cylindrical muffle of 4 inches inside diameter and inches long, in which is a quartz tube of 3.5 inches ID. and 72 inches long (this tube sticks out at both ends of the muffle) with a small tubulation for introducing the gas at one end and a big flange at the other end for introduction of the wafers, which were stacked in a quartz boat. The cool oven was first purged with a nitrogen flow of about [2 liters/hour for 30 minutes.

Then the nitrogen flow was interrupted and a hydrogen flow of the same magnitude l0 to 15 liters/hour) established through the furnace tube. As soon as the hydrogen had filled the tube (ten minutes), the heater of the oven was turned on and the temperature brought to 330C 35" in about one-half hour, and maintained at that level for sixteen hours, while the flow of hydrogen was maintained constant. Then the heater was turned off. After two hours, at a temperature near 270 C, the hydrogen was in turn replaced by nitrogen, and after 30 minutes, the boat containing the wafers was pulled to the cooler front end of the oven, where they cooled to room temperature in one to two hours, before being removed from the oven. Then the wafers, with their now semiconductive holes, were given their fingerpattern electrodes by vacuum evaporation through a photoresist mask. They were then ready to be mounted in a scanning multiplier device.

I claim:

1. A method of forming dynode plates comprising:

a. providing a plate of insulating material having a plurality of holes therethrough,

b. selecting a glazing composition comprised of a lead glass having 30-60 mole percent lead oxide, about 6 mole percent bismuth trioxide, about 9 mole percent silicon dioxide, and 30 mole percent boron trioxide,

c. adding liquid to said glazing composition to obtain i a suspension between L0 and L30,

. dipping said plate in said suspension to coat at least the inside of the holes therethrough with said glazing composition,

e. drying said glazing composition at ambient temperature,

f. heating said plate with the dried glazing composition thereon at a temperature of 500 to 550 C to fuse said glazing composition into a glaze, and

g. heating said plate with said glazed composition thereon at a temperature of 325 to 350 C in a reducing atmosphere to provide a semiconductive layer on said glazed composition.

2. The method of forming dynode plates claimed in claim 1 wherein said selected glazing compound is divided into fine solid particles less than 5 microns, heating the glazing compound to the melting point, pouring the hot melt into water to form frit, drying the frit, and milling the frit to a particle size of less than 5 microns.

3. The method claimed in claim 2 where the reducing atmosphere is hydrogen.

4. The method claimed in claim 3 wherein reducing heating is for a period of about 16 hours. 

1. A METHOD OF FORMING DYNODE PLATES COMPRISING: A. PROVIDING A PLATE OF INSULATING MATERIAL HAVING A PLURALITY OF HOLES THERETHROUGH, B. SELECTING A GLAZING COMPOSITION COMPRISED OF A LEAD GLASS HAVING30-60 MOLE PERCENT LEAD OXIDE, ABOUT 6 MOLE PERCENT BISMUTH TRIOXIDE, ABOUT 9 MOLE PERCENT SILICON DIOXIDE, AND 30 MOLE PERCENT BORON TRIOXIDE, C. ADDING LIQUID TO SAID GLAZING COMPOSITION TO OBTAIN A SUSPENSION BETWEEN 1.0 AND 1.30, D. DIPPING SAID PLATE IN SAID SUSPENSION TO COAT AT LEAST THE INSIDE OF THE HOLES THERETHROUGH WITH SAID GLAZING COMPOSITION, E. DRYING SAID GLAZING COMPOSITION AT AMBIENT TEMPERATURE, F. HEATING SAID PLATE WITH THE DRIED GLAZING COMPOSITION THEREON AT A TEMPERATURE OF 500* TO 550*C TO FUSE SAID GLAZING COMPOSITION INTO A GLAZE, AND G. HEATING SAID PLATE WITH SAID GLAZED COMPOSITION THEREON AT A TEMPERATURE OF 325* TO 350*C IN A REDUCING ATMOSPHERE TO PROVIDE A SEMICONDUCTIVE LAYERON SAID GLAZED COMPOSITION.
 2. The method of forming dynode plates claimed in claim 1 wherein said selected glazing compound is divided into fine solid particles less than 5 microns, heating the glazing compound to the melting point, pouring the hot melt into water to form frit, drying the frit, and milling the frit to a particle size of less than 5 microns.
 3. The method claimed in claim 2 where the reducing atmosphere is hydrogen.
 4. The method claimed in claim 3 wherein reducing heating is for a period of about 16 hours. 